linux-kernel - Re: [RFC PATCH v1 0/5] nolibc x86-64 string functions

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Message-ID: <ZPH4I2W+zvhK93U/@biznet-home.integral.gnuweeb.org>
Date:   Fri, 1 Sep 2023 21:41:39 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Willy Tarreau <w@....eu>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Nicholas Rosenberg <inori@...x.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Michael William Jonathan <moe@...weeb.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 0/5] nolibc x86-64 string functions

On Fri, Sep 01, 2023 at 02:23:28PM +0000, David Laight wrote:
> From: Ammar Faizi
> > Sent: 01 September 2023 14:06
...
> > __asm__ (
> > ".section .text.nolibc_memmove\n"
> > ".weak memmove\n"
> > "memmove:\n"
> > "    movq    %rdx, %rcx\n"
> > "    movq    %rdi, %rdx\n"
> > "    movq    %rdi, %rax\n"
> 
> You seem to have confused yourself about whether you are using %eax or %edx.

What do you mean? They're all 64-bit pointers.

What I know is that the %rdx will be clobbered by "subq %rsi, %rdx"
below and the %rax should be return value. That's why I copy the %rdi
twice. memmove() returns the dst pointer. Did I miss something?

> > "    subq    %rsi, %rdx\n"
> > "    cmpq    %rcx, %rdx\n"
> > "    jnb     .Lforward_copy\n"
> 
> I think I'd fall through to the forwards copy
> and not worry about replicating the 'reps movsb' and 'ret'.
> IIRC 'cld' can be slow as well.

Alright, I will avoid cld for the forward copy.

> > "    leaq    -1(%rdi, %rcx, 1), %rdi\n"
> > "    leaq    -1(%rsi, %rcx, 1), %rsi\n"
> > "    std\n"
> > ".Lforward_copy:\n"
> > "    rep movsb\n"
> > "    cld\n"
> > "    ret\n"
> > );

-- 
Ammar Faizi